Implantable devices may be employed in various applications. For example, an implantable sensing device may perform functions, such as sensing blood pressure, sensing cardiac signals, sensing neurological signals, and so on. In addition, an implantable stimulation device may perform functions, such as generating cardiac stimulation signals, generation neurological stimulation signals, and so on.
Advances in integrated circuit technology have resulted in the availability of very small, very low power, high density integrated circuits. This, in turn, has made it possible to produce very small implantable medical devices such as, for example, sensors and satellite pacers.
Although the amount of power required for such devices is typically very small, external power techniques may be employed for these devices to enable the use of relatively small batteries or no batteries at all. Accordingly, in some implementations, external power is transferred to an implanted medical device via an electromagnetic field generated by an external device. For example, power may be transferred from an external device to recharge a rechargeable battery of an implanted medical device. As another example, for implanted medical devices that obtain all power solely from an external device, power may be transferred from an external device to power the implanted medical device.
In practice, it may be relatively difficult to couple power to a small implantable medical device. This is because an externally applied electromagnetic field tends to be weakly coupled to an implanted medical device. To couple an external source of energy to an implanted medical device, a large external coil may be used to create a strong electromagnetic field over a large area. However, only a small portion of this field will pass through the implanted medical device. For example, in some cases, the coupling coefficient is on the order of 0.01%.